A digital Vertical Interval Time and Control Code (VITC) is an American National Standards Institute (ANSI) standard code which is inserted into two non-consecutive lines of the vertical interval of a video signal. In the video program production industry, the VITC is normally non-return to zero (NRZ) modulated and inserted in analog video signals to provide an index for video editing and multi-tape synchronization.
Traditionally, the VITC code is recovered with a threshold detector set to switch at half the amplitude of the digital VITC signal. More sophisticated approaches compensate for signal amplitude variations and distortion by varying the switching level of the threshold detector, but the level is still maintained at a constant level throughout a given video line.
Unfortunately, the video recording process utilized on many recorder formats such as U-Matic, VHS, or Beta requires filtering of the video signal into its luminance and chrominance components. This process restricts the luminance bandwidth to about 2.7 Megahertz (3 Decibel point).
When a video signal containing a VITC code is recorded and played back, it is slightly distorted as a result of the band limitation described. Although this distortion may not be significant in a first-generation recording, the buildup of distortion after as few as three re-recordings may prevent recovery of the VITC code using traditional techniques.
Many prior art systems have been developed to recover VITC or similar control codes, but these systems are generally complex and not adapted to recover NRZ data which has been directly recorded without the benefit of a modulation scheme. For example, U.S. Pat. No. 3,987,484 to R.P. Bosche et al discloses a circuit for outputting NRZ data from an input signal. This circuit includes threshold detectors which detect whether an input amplitude of an audio signal is greater than twenty-five and seventy-five perdent, respectively, of the full amplitude of the carrier. The function of the circuit is to recover NRZ data from a signal where the data is modulated/encoded as a series of tone bursts with three amplitude levels (zero, 50% and 100%). The circuit utilizes threshold detectors feeding retrigerable oneshots to filter out the modulating carrier and a flip-flop at the output is used to store the last detected logic level.
U.S. Pat. No. 4,210,785 to W.B. Huber et al discloses a tape replay system which uses digital filtering techniques to implement a frequency discriminator to recover binary data which has been encoded using two frequencies. Each binary bit of data is encoded as a burst as two frequencies such that a binary '0' is represented by a cycle of a high frequency followed by a few cycles of a lower frequency and a binary '1' is represented by a number of cycles of the high frequency followed by cycles of the lower frequency. The circuit uses edge detector and flip-flop elements, but is not a simple circuit adapted to recover a VITC data signal which has been subject to phase and/or amplitude distortion and which is not uniform over the frequency spectrum occupied by the signal.
Finally, U.S. Pat. No. 4,167,028 to R. Tobey discloses a method and apparatus for time signal encoding/decoding in a tape replay system where the purpose of the circuit is to decode data which is encoded as a pulse width modulation of the control track pulses recorded on video tape. The encoded data signal is within the bandwidth of the system which employs Schmidt trigger and flip-flop elements in a decoder.
In view of the known prior art, it is apparent that there is a need for a simple method of accurately recovering VITC code signals from analog video signals when the VITC signals are distorted.